Labour has held the seat comfortably since 1997 but the party is expected to pay a heavy price for the MPs' expenses controversy in the first Westminster byelection since the Commons was rocked by the scandal.

David Cameron is due to visit the constituency for the sixth time this morning, giving a final boost to a campaign seen by Conservative headquarters as an important test of the party's ability to withstand a Labour attack based on a "Tory cuts" message.

Unusually, the votes will be counted tomorrow rather than at the close of the polls this evening, partly because staffing a daytime count is easier. This has not happened at a byelection in recent years.

The byelection was caused by the resignation of Ian Gibson, a leftwinger who quit parliament after Labour ruled that he would not be allowed to stand at the next election because he used parliamentary expenses to fund a flat which he subsequently sold at a discount to his daughter.

Gibson, who was popular in the constituency, had a majority of 5,459 in 2005, and Labour's decision to ban him as a candidate appears to have backfired, with some voters telling the party that they will not vote for his replacement, 28-year-old Chris Ostrowski, because they think Gibson was treated unfairly.

The Conservatives seem confident of victory. But they are nervous of comparisons with the Crewe and Nantwich byelection last year, when the Tories overturned a Labour majority of more than 7,000, winning by 7,860 with a swing of 17.6%.

"Crewe and Nantwich took place against the backdrop of the abolition of the 10p rate of tax and voters were so angry that they came straight over to us. Norwich North is different because, as a result of expenses, the voters are angry with all parties," said one senior Tory.

Chloe Smith, the 27-year-old Conservative candidate, has responded to the challenge of campaigning in a climate of scepticism about politicians by issuing her own "contract with the people of Norwich North" containing various promises on policy and expenses.

The Liberal Democrats, who were well behind the Tories in 2005, claimed yesterday that it was now a Tory-Lib Dem contest, and that Labour could come third behind their candidate, April Pond.

At the start of the byelection, Labour campaigned aggressively on the theme of "Tory cuts", in what was seen as a dry run for the general election strategy being planned by Brown. But the Tories believe that this tactic has been unsuccessful in Norwich North because they are winning the argument on public spending nationally.

Labour's campaign suffered a blow when Ostrowski was taken to hospital with swine flu yesterday. He was recuperating today, but cabinet ministers Andy Burnham and Alan Johnson were in Norwich North campaigning on his behalf.

"I am very confident that we can win this byelection," said Burnham. Privately, Labour was trying to make life difficult for Cameron by suggesting that anything less than a 10,000 majority would be a disappointment for the Tory leader.